Abstract

Most animals actively avoid predators. If such a reaction reduces competitive ability, for example by reducing food intake, predator presence can lead to trait-mediated indirect effects. Because predator avoidance typically leads to reduced growth rather than reduced survival, itseffect on population processes is difficult to assess. This is especially true for organisms with complex life-cycles, where predator avoidance during one stage is expected to lead to traitmediated indirect effects if it has effects reaching into the following life stages. I experimentallyinvestigated the effect of caged (thus non-lethal) dragonfly larvae on the competition betweentadpoles of two frog species (Rana lessonae and R. esculenta) and on juvenile frog survival during the subsequent terrestrial stage. In response to caged predators, R. lessonae delayed metamorphosis more than R. esculenta, but they both metamorphosed heavier. Thesedifferences suggest the possibility of a competitive disadvantage for R. lessonae in the presence of predators, which could lead to trait-mediated indirect effects. However, the presence of predators did not modify competitive effects and had no measurable consequences on terrestrial survival. Regardless of the presence of predators, competition during the larval stage had large effects on metamorphosis and led to strongly decreased survival in the subsequent terrestrial stage. These results suggest that trait-mediated indirect effects are not important in this system, because the predator reaction of the tadpoles in both species had no measurable effect on the following life stage and, therefore, probably no strong effect on community dynamics.